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1.
ACS Nano ; 10(3): 3580-8, 2016 Mar 22.
Artigo em Inglês | MEDLINE | ID: mdl-26866442

RESUMO

When designing semiconductor heterostructures, it is expected that epitaxial alignment will facilitate low-defect interfaces and efficient vertical transport. Here, we report lattice-matched epitaxial growth of molybdenum disulfide (MoS2) directly on gallium nitride (GaN), resulting in high-quality, unstrained, single-layer MoS2 with strict registry to the GaN lattice. These results present a promising path toward the implementation of high-performance electronic devices based on 2D/3D vertical heterostructures, where each of the 3D and 2D semiconductors is both a template for subsequent epitaxial growth and an active component of the device. The MoS2 monolayer triangles average 1 µm along each side, with monolayer blankets (merged triangles) exhibiting properties similar to that of single-crystal MoS2 sheets. Photoluminescence, Raman, atomic force microscopy, and X-ray photoelectron spectroscopy analyses identified monolayer MoS2 with a prominent 20-fold enhancement of photoluminescence in the center regions of larger triangles. The MoS2/GaN structures are shown to electrically conduct in the out-of-plane direction, confirming the potential of directly synthesized 2D/3D semiconductor heterostructures for vertical current flow. Finally, we estimate a MoS2/GaN contact resistivity to be less than 4 Ω·cm(2) and current spreading in the MoS2 monolayer of approximately 1 µm in diameter.

2.
ACS Nano ; 9(2): 2080-7, 2015 Feb 24.
Artigo em Inglês | MEDLINE | ID: mdl-25625184

RESUMO

Tungsten diselenide (WSe2) is a two-dimensional material that is of interest for next-generation electronic and optoelectronic devices due to its direct bandgap of 1.65 eV in the monolayer form and excellent transport properties. However, technologies based on this 2D material cannot be realized without a scalable synthesis process. Here, we demonstrate the first scalable synthesis of large-area, mono and few-layer WSe2 via metal-organic chemical vapor deposition using tungsten hexacarbonyl (W(CO)6) and dimethylselenium ((CH3)2Se). In addition to being intrinsically scalable, this technique allows for the precise control of the vapor-phase chemistry, which is unobtainable using more traditional oxide vaporization routes. We show that temperature, pressure, Se:W ratio, and substrate choice have a strong impact on the ensuing atomic layer structure, with optimized conditions yielding >8 µm size domains. Raman spectroscopy, atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (TEM) confirm crystalline monoto-multilayer WSe2 is achievable. Finally, TEM and vertical current/voltage transport provide evidence that a pristine van der Waals gap exists in WSe2/graphene heterostructures.

3.
Nat Commun ; 5: 5246, 2014 Nov 18.
Artigo em Inglês | MEDLINE | ID: mdl-25404060

RESUMO

Monolayer molybdenum disulfide (MoS2) has attracted tremendous attention due to its promising applications in high-performance field-effect transistors, phototransistors, spintronic devices and nonlinear optics. The enhanced photoluminescence effect in monolayer MoS2 was discovered and, as a strong tool, was employed for strain and defect analysis in MoS2. Recently, large-size monolayer MoS2 has been produced by chemical vapour deposition, but has not yet been fully explored. Here we systematically characterize chemical vapour deposition-grown MoS2 by photoluminescence spectroscopy and mapping and demonstrate non-uniform strain in single-crystalline monolayer MoS2 and strain-induced bandgap engineering. We also evaluate the effective strain transferred from polymer substrates to MoS2 by three-dimensional finite element analysis. Furthermore, our work demonstrates that photoluminescence mapping can be used as a non-contact approach for quick identification of grain boundaries in MoS2.

4.
ACS Nano ; 8(8): 7930-7, 2014 Aug 26.
Artigo em Inglês | MEDLINE | ID: mdl-25019978

RESUMO

Semiconducting MoS2 monolayers have shown many promising electrical properties, and the inevitable polycrystallinity in synthetic, large-area films renders understanding the effect of structural defects, such as grain boundaries (GBs, or line-defects in two-dimensional materials), essential. In this work, we first examine the role of GBs in the electrical-transport properties of MoS2 monolayers with varying line-defect densities. We reveal a systematic degradation of electrical characteristics as the line-defect density increases. The two common MoS2 GB types and their specific roles are further examined, and we find that only tilt GBs have a considerable effect on the MoS2 electrical properties. By examining the electronic states and sources of disorder using temperature-dependent transport studies, we adopt the Anderson model for disordered systems to explain the observed transport behaviors in different temperature regimes. Our results elucidate the roles played by GBs in different scenarios and give insights into their underlying scattering mechanisms.

5.
Nat Mater ; 13(8): 786-9, 2014 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-24776537

RESUMO

The crystal structure of a material plays an important role in determining its electronic properties. Changing from one crystal structure to another involves a phase transition that is usually controlled by a state variable such as temperature or pressure. In the case of trilayer graphene, there are two common stacking configurations (Bernal and rhombohedral) that exhibit very different electronic properties. In graphene flakes with both stacking configurations, the region between them consists of a localized strain soliton where the carbon atoms of one graphene layer shift by the carbon-carbon bond distance. Here we show the ability to move this strain soliton with a perpendicular electric field and hence control the stacking configuration of trilayer graphene with only an external voltage. Moreover, we find that the free-energy difference between the two stacking configurations scales quadratically with electric field, and thus rhombohedral stacking is favoured as the electric field increases. This ability to control the stacking order in graphene opens the way to new devices that combine structural and electrical properties.

6.
Nanotechnology ; 25(15): 155702, 2014 Apr 18.
Artigo em Inglês | MEDLINE | ID: mdl-24642948

RESUMO

We have studied temperature-dependent (77-300 K) electrical characteristics and low-frequency noise (LFN) in chemical vapor deposited (CVD) single-layer molybdenum disulfide (MoS2) based back-gated field-effect transistors (FETs). Electrical characterization and LFN measurements were conducted on MoS2 FETs with Al2O3 top-surface passivation. We also studied the effect of top-surface passivation etching on the electrical characteristics of the device. Significant decrease in channel current and transconductance was observed in these devices after the Al2O3 passivation etching. For passivated devices, the two-terminal resistance variation with temperature showed a good fit to the activation energy model, whereas for the etched devices the trend indicated a hopping transport mechanism. A significant increase in the normalized drain current noise power spectral density (PSD) was observed after the etching of the top passivation layer. The observed channel current noise was explained using a standard unified model incorporating carrier number fluctuation and correlated surface mobility fluctuation mechanisms. Detailed analysis of the gate-referred noise voltage PSD indicated the presence of different trapping states in passivated devices when compared to the etched devices. Etched devices showed weak temperature dependence of the channel current noise, whereas passivated devices exhibited near-linear temperature dependence.

7.
Nano Lett ; 13(4): 1541-8, 2013 Apr 10.
Artigo em Inglês | MEDLINE | ID: mdl-23470052

RESUMO

The growth of large-area bilayer graphene has been of technological importance for graphene electronics. The successful application of graphene bilayers critically relies on the precise control of the stacking orientation, which determines both electronic and vibrational properties of the bilayer system. Toward this goal, an effective characterization method is critically needed to allow researchers to easily distinguish the bilayer stacking orientation (i.e., AB stacked or turbostratic). In this work, we developed such a method to provide facile identification of the stacking orientation by isotope labeling. Raman spectroscopy of these isotopically labeled bilayer samples shows a clear signature associated with AB stacking between layers, enabling rapid differentiation between turbostratic and AB-stacked bilayer regions. Using this method, we were able to characterize the stacking orientation in bilayer graphene grown through Low Pressure Chemical Vapor Deposition (LPCVD) with enclosed Cu foils, achieving almost 70% AB-stacked bilayer graphene. Furthermore, by combining surface sensitive fluorination with such hybrid (12)C/(13)C bilayer samples, we are able to identify that the second layer grows underneath the first-grown layer, which is similar to a recently reported observation.


Assuntos
Grafite/química , Nanoestruturas/química , Análise Espectral Raman , Gases/química , Marcação por Isótopo , Propriedades de Superfície
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